Making tracks: electronic excitation roles in forming swift heavy ion tracks

Abstract

Swift heavy ions cause material modification along their tracks, changes primarily due to theirvery dense electronic excitation. The available data for threshold stopping powers indicatetwo main classes of materials. Group I, with threshold stopping powers above about10 keV nm−1, includes some metals, crystalline semiconductors and a few insulators. Group II,with lower thresholds, comprises many insulators, amorphous materials and highTc oxide superconductors. We show that the systematic differences in behaviourresult from different coupling of the dense excited electrons, holes and excitonsto atomic (ionic) motions, and the consequent lattice relaxation. The couplingstrength of excitons and charge carriers with the lattice is crucial. For group II, themechanism appears to be the self-trapped exciton model of Itoh and Stoneham (1998Nucl. Instrum. Methods Phys. Res. B 146 362): the local structural changes occurroughly when the exciton concentration exceeds the number of lattice sites. Inmaterials of group I, excitons are not self-trapped and structural change requiresexcitation of a substantial fraction of bonding electrons, which induces spontaneouslattice expansion within a few hundred femtoseconds, as recently observed bylaser-induced time-resolved x-ray diffraction of semiconductors. Our analysis addresses anumber of experimental results, such as track morphology, the efficiency of trackregistration and the ratios of the threshold stopping power of various materials.